The night sky has an overall background luminosity.
We are aware of the localized
sources of light in a moonless night sky (the stars and planets, the
zodiacal light, and gegenschein), but in addition to the astronomical
sources there is an overall uniform luminosity originating from the
Earth's own atmosphere. We are not normally aware of this airglow
because it is so uniform.
It is the combination of astronomical and airglow sources that allows
us to see the silhouette of an object held against the "dark" sky on a
clear moonless night.

Airglow refers to the luminosity (glowing) of the atmosphere

The brightest region of airglow is about 10 mile (10 to 20 km) thick zone at an altitude of
about 60 miles (100 km). One contributor to airglow is the sodium layer.

Airglow viewed from space. The orangish arc extending to the left
from the aurora is airglow viewed edgewise on the limb of the Earth.

Another nice shot of airglow visible on the horizon and the lights along the Nile in the center of the picture

Airglow: The Movie Video and commentary from the Int'l Space Station by astronaut Don Pettit and another nice video
Also take a look at NASA's Space Station Science Picture of the Day for April 11, 2003 and another and another nice shot (scroll down to second picture)

Airglow is commonly divide into:

Dayglow (when entire atmosphere is illuminated by the Sun)
is the brightest airglow due to the importance of RESONANT and
FLUORESCENT processes (see below) but it is overwhelmed by direct
and scattered sunlight

Twilightglow (when only the upper atmosphere is illuminated)
is the most readily observable airglow from the ground since the observer is in
darkness (and Rayleigh scattering of sunlight by the dense lower
atmosphere is absent) while the airglow region of upper atmosphere
is still illuminated

Nightglow (when entire atmosphere is in darkness) is
not as bright as dayglow since CHEMILUMINESCENCE (see
below) is the dominant process; however contributes more light
than starlight to the total luminosity of the night sky

Airglow vs aurora

spatial extent: global vs. high latitude

luminosity: relatively uniform vs. highly structured

energy source: solar radiation vs. solar wind

Airglow is due to emission from excited states formed by processes resulting
(directly or indirectly) from solar radiation. These processes include:

RESONANCE: emitted light is at same color as that absorbed resulting from excitation by the absorption of solar radiation

FLUORESCENCE: emitted light is at lower frenquency, i.e. a different color, resulting from excitation by the absorption of solar radiation

PHOTOIONIZATION: emitted light is from the excited states of ionized fragments caused by solar radiation

PHOTODISSOCIATION: emitted light is from the excited states of neutral fragments caused by solar radiation

INELASTIC COLLISIONS: emitted light results from excitation caused by the impact of high energy ("hot")
electrons that are produced in photoionization [NOTE: This is the same
process that causes the luminous aurora but in that case the "hot" electrons
come from the magnetosphere through interactions with the solar wind.]

CHEMILUMINESCENCE: emission results from chemical reactions mainly
between oxygen and nitrogen atoms and molecules and hydroxyl molecules at a
height between 100 and 300 kilometers. Solar radiation energy breaks molecules
apart during the day, and it is their recombination, which is accompanied
by the emission of light, that generates the nightglow.

EXCITATION by COSMIC RAYS (high energy radiation and particles from outside the solar
system) make a small contribution to airglow as well.

Sometimes airglow is not so uniform.

Gravity waves generated by strong thunderstorms or, as pictured in the link, strong winds encountering mountains have a rippling effect in the upper atmosphere like a stone thrown into a pond.

During magnetic storms the airglow at mid-latitudes is
modified by heating effects, producing stable auroral red (SAR) arcs(as seen on the right edge of the image).
These are broad diffuse arcs of light that align along
geomagnetic parallels between about 40-55 degrees and located at a height of
300-400 kilometers. They are quite bright in red oxygen emission but
are subvisual because the human eye's sensitivity at that wavelength
is only about 5% of its peak sensitivity in the yellow region of the
spectrum.

Brief Historical Outline

1868

Anders Angstrom discovers green line is present in the night sky even when no aurorae are present

1920's

Robert John Strutt (4th Baron Rayleigh) begins investigations
[Note: he is referred to as the "airglow Rayleigh"; his father
John William Strutt, 3rd Baron Rayleigh, is the "scattering Rayleigh"]

1923

John McLennon & G.M. Shrum identify green line to be due to atomic oxygen